A naked cat
You should use a meter stick to measure the length of a soccer pitch.
I would use a centimetre to measure the length of a boy. Not sure what a boy yard is, though.
Length is measured in meters in the metric system.
Microscope
Meters if you're using the metric system. Otherwise use feet.
inches or centimeters
Meters should be used to measure the length of a tennis court in metric units.
use a ruler to measure the sides then do length times width
we don't use length. we use liter.
Any sensibly sized unit of length, such as feet, yards or metres.
If you live in the United States, you should use inches. If you live outside of the US, you should use centimetres.
If you are able to use fractions - or smaller units - there is never a need to do so.